Method of processing mica



March 21, 1944. I BARLETT 2,344,670

METHOD OF PROCESSING MICA Filed Jan. 29, 1942 2 Sheets-Sheet 1 49, fag 2, I attornegs March 21,1944. BARLETT 2,344,670

' METHOD OF PROCESSING MICA Filed Jan. 29, 1942 2 Sheets-Sheet 2 Pl/OTO C'EL L POLAROID I8 M POLAROID Q 5/5/51 2 filaizfaaz g I 31! W I attorneys weakness from Patented Mar. 21 1944 UNITED STATES PATENT OFFICE METHOD OF PROCESSING MIOA BeienBlair Barlett,

Flint, Mich aseignor to Genon, Detroit, Mich, a cor-' Application January 2a, 1942. Serial No. m

11 Claim.

This invention relates to a method of orienting mica to permit bending or rolling the same with a minimum of cracking. Specifically, it relates to a method of determining the optimum rolling direction of sheet mica.

Sheet mica has a considerable amount of flexibility which may vary according to the composition of the mica or the source from which it is mined. Sheet mica can be rolled about a cylinder having a comparatively short radius but, as is known in the art, the mica is subject to cracking when so rolled.

It is an object of this invention to provide a method of orienting the mica so that it may be rolled about a cylinder the axis of which is aligned with the strongest, or leastweak, of the axes of weakness. This orientation of the mica is accomplished by a determination of the axes of sion figures of the mica. Polariaed light may be used to aid in finding the axis of greatest weakness.

Figure 1 of the showing-of the patterns formed on mica known to the art as the pressure figure and the percussion figure.

Figure 2 shows a block of mica, or book," as it is known in the art.

drawings is a diagrammatic w The pressure figure pressed against the mica sheet while the latter rests on asomewhat resilient surface, another figure known as the pressure Ifigure is produced. is similarto the percussion figure in that it may be present with as many as six rays but it more often has fewer than all six of the rays. If several percussion and pressure figures are formed on the sheet, it will be the pressure figures and percusfound that all the percussion figures will be oriented line for line in substantially the same direction, and all the pressure figures will be oriented line for line in substantially the same direction. As has been pointed out, not all of the rays or lines are likely to be present in each figure so that some figures may have rays or lines not present in other figures. It will further be found that rays of the percussion figures will form angles with the rays of the pressure figures which are whole multiples of substantially 30.

In Figure 1, I have shown a perfect pressure figure and a perfect percussion figure which for convenience are disposed about the same center. For the sake of convenience, the solid lines a, b

l and 0 may be taken to be the rays of the pres- Figure 3 shows a sheet of mica which is being stressed to form a pressure figure.

Figure 4 is a perspective view of the apparatus which is used to produce polarized light,

Figure 5 is a. side elevation view of the apparatus shown in Figure 4. a

Figure 6 is a view of the working surfaceof the polarizing apparatus, showing the lines made on thepolarizing material. a

Figure '7 shows a "cigarette such as might be used in spark plug manufacture.

Figure 8 is a diagrammatic showing of a photoelectric system for more accurately determining the axis of greatest weakness.

As is known in the art, when a sheet of mica is struck by a sharp object, such as a nail, what is known as a percussion figure is produced. This figure consists of lines radiating away from the point at which the mica is struck. The lines may constitute a figure having as many as six rays, 1. e., three intersecting lines, each of which .forms two rays. More often, however, fewer than six of the rays appear. When all six rays are present they are substantially 60 apart. Sometimes only three of the rays appear and sometimes even only two. If, instead of striking of rolled mica sure figure. The dotted lines :0, y and a represent the rays of the percussion figure. The angles 0 are substantially 60, and the angles are substantially 30. 1, that each ray or line of apercussion figure substantially bisects the angle included between the two adjacent pressure lines. It will generally be found that one of the percussion lines, or axes, as they may be called, will be stronger or more pronounced than the other two. This line or axis may be called the principal line or principal axis. For convenience let us designate line :r-a: as the principal axis in Figure 1.

I have found, that the lines comprising the pressure and percussion figures are definitely related to axes of weakness of sheet mica in bending. For example, where it is desired to bend or roll sheet mica about a cylindrical axis, ,1 have found that it is highly desirable, to avoid cracking of the mica sheet,'to so orient the mica that the axis of the cylinder about which the sheet is bent is ot parallel to. one of the pressure axes or lines. A further condition, however, should preferably be observed, namely, the axis of the cylinder about which the mica sheet is bent should not be parallel to the major or principal percussion axis. It appears, therefore, from a study of Figure 1, that the axis of the cylinder about which the mica is bent should be approximately parallel to percussion axis 11- 1 the mica with a sharp point, a blunt point is so or percussion axis z-z. If a strip or sheet of It, is evidentfrom Figure 7 weakness without the aid of mica be rolled about axis yy or H, two of the pressure axes will wind about the cylinder to form spiral helices and the third will form a simple spiral. Thus the rolling will fulfill the requirement as to the pressure axes in that the cylinder about which the mica is rolled will not be parallel to any pressure axis. It will also be noted that the major percussion axis :r-x will wind about the cylinder in a-spiral helix. Therefore, rolling the mica in this direction also-fulfllls the requirement as to the major percussion axis, in that the axis of the cylinder about; which the sheet is rolled is not parallel to said major I percussion axis.

In Figure 2 is shown a block of mica. Hi which is known in the art as a book. This book or block may be split indicated at H. much as all the sheets of one book have the same crystalline orientation and the same general outline or contour, locating the axes of weakness for one of the sheets will serve to orient all the sheets of that book. If, then,'one sheet is oriented according-to the analysis set forth above, all the sheets of the bookcan be rolled about the strongest, or least weak, of the axes of weakness. The axes of weakness may be determined in the usual manner, namely, by striking the sheet with a sharp point to produce one or more percussion figures and by dull instrument while the sheet is supported on a somewhat resilient surface to form one or more pressure figures. The major percussion axis will not always be identifiable from the percussion figures, and may be determined with the aid or pressing the sheet with a up into individual sheets as It will be understood that, inastus will preferably .of light polarizing material is to keep it from being scratched. Current for bulbs 20 maybe provided from any suitable source, not shown, through a cord or cable 24. It will usually be found expedient to mount another sheet of light polarizing material above the first in such a manner that the observer looks through the upper sheet onto the working surface which comprises the lower sheet of light polarizing material or its protective cover. This second sheet of light polarizing material may be secured in a frame 28 supported by rods 30 on box M.

The observer using my light polarizing apparaface it from the left; as seen in Figures 4 and 5. Light polarizing material 25 will preferably be disposed above material l8 in such a manner as to provide a viewing angle of approximately 75". It should be understood, however, that this angle is not critical. As a matter of fact, the pieces of light polarizing material should be of sulficient size to permit some variation in the viewing angle. Where the piece substantially square, it will generally be found thata square 6 a petrographic microscope. If desired, orientation of all the axes of weakness may be obtained by the use of such a microscope. However, petrographic microscopes are expensive pieces of equipment and it is an object of this invention to provide a method of determining the axes of such a microscope. Orientation of sheet mica is easily effected according to this invention by first forming one or more pressure figures. I have found that pressure figures may be formed in the mica by crimping a sheet of the mica in the hands in much the same manner as one would fold a sheet of .paper to form a cone thereof This operation is shown in Figure 3. It is not necessary to form with 60 lines I,

percussion figures to orient the mica according to this invention, although the invention 'does contemplate orientation of the mica according to,the axes of weakness as would be indicated by precusslon figures. Such figures-may be formed if desired in the practiceof the invention. In order that numerous pressure lines may be formed, the mica sheet may becrimped in several places to form several pressure figures. When this has been done, the sheet of mica may conveniently be exposed to polarized light. To this end, I provide apparatus which takesthe form shown in Figures 4 and 5 in which a box II is provided with an opening l6 substantially centrally of its top face. In opening it I dispose a sheet of material I! which is capable of polarizing light waves, such as the substance which is commercially known as Polaroid. One or more conventional light bulbs 20 may be provided to furnish the light for the orientation process. Light diffusing means 21 are preferably provided to insure that an even, diffused light be directed to the light polarizing. means. A substantially transparent cover, not shown, such as clear glass, may be placed over the light polarizing material h sloping to the right, as seen in Figure portant. It will sufiice to have enough to 8 inches on a side will work satisfactorily. Light polarizing piece 26 may be from 10 to 14 inches above the piece l8. It should be understood, however, that these figures are merely examplesand that other dimensions: may be used. The light polarizing material will preferably be so placed that the direction of maximum light transmission therein will be at 45 to the 'forward'edge of the viewing apparatus with thedirection line sloping toward the left, as represented by dotted line din Figure 6. The direction of total absorption will then be the direction of dotted -line e in Figure 6 at right angles to line (1. This position applies to both pieces of light polarizing nifiterial. In the preferred form of my invention, th lower piece l8 of light polarizing material is provided a and h, as seen inFigure 6. Lines f are parallel to the front edged the viewing box and lines 9 and h are drawn at with lines I, the lines 9 sloping to the left and the lines 6. The not im- 01 such lines to permit easy alignment therewith of the lines of the pressure figures of the mica sheet being oriented.

spacing and the'number of these lines is In orienting mica according to the method of this invention a sheet of mica is split from a book, as shown in Figure 2. This sheet is then crimped, as seen in Figure 3, so as to form one or more pressure figures. The sheet is then placed on polarizing material ll and is rotated, while exposed to the polarized light passing through material l8, to the position where it appears darkest-4. e. to the position where a minimum of polarized light passes through the mica sheet. For a full 360 rotation of such a sheet the observer will find that there are two positions of the mica in which it appears somewhat darkened or colored and there are-two positions of the mica through which still less light is passed. The two positions of greater light transmissibility are 180 apart and substantially at right angles to these positions are the two positions of minimum light transmissibility which are, 01' course, also 180 apart. The observer rotates the mica to either position of minimum light transmlssibility. Experience will show that there is a rather wide angle'in which the mica appears dark and that it is extremly difiicuit'to locate exactly the position of minimum light transmission. However, after back edge of opening as shown in Figure approximately the poslthe observer has located tion of minimum light transmission he should look at the pressure iigures previously formed on the mica sheet. It will be found that the lines of the pressure figures are very nearly parallel to the 60 lines previously formed on the light polarizing material surface. until it is oriented so that the pressure axes are aligned perfectly with the 60 lines I, g and h, the observer or operator will orient the piece of mica to its exact minimum light transmission p sition. The mica sheet will then be oriented so that the major percussion axis will extend in a straight line away from the observer- -i. e. the major percussion axis will then be perpendicular to the front or forward edge of the light polarizing box. Having determined the direction of the major percussion axis, the observer can then mark the sheet for cutting into strips, the long axes of which are parallel eitherto'lines g or h. The choice of direction will be determined to some extent by the contour of the sheet being oriented. It is naturally desirable to cut the mica into the longest strips orientation according to this analysis. It will generally be found that cutting the mica into strips parallel to one of the lines 9 or it will produce longer strips than cutting it along the other line. The direction giving the longest strips will, of course, be the preferred direction for cutting. It is understood, of course, that these instructions as to cutting apply to the usual situation in which the long axis of the mica strip is perpenconsistent with proper By adjusting the sheet of the major percussion axis. Having determined this direction, the observer or operator working with the mica will proceed substantially as outlined above .to determine the direction of the. axis of the cylinder about which or rolled for minimum cracking.

. With some grades of mica it may be found that the direction of the major percussion axis relative to the direction of the cylinder about which the dicular to the axis of the cylinder about which the strip is rolled. If desired, a suitable straight edge, not shown, may be pivoted at one end on the top of box l4 near either the front edge or it in such a manner that it may be rotated by movement of its free end so as to permit marking of the mica sheet along the direction of either lines a or lines In, Alternatively, the straight edge may be fastened securely at both ends along the direction of lines 9 or lines h, in which case, if it be found that the long axis of the mica sheet does not extend in the same general direction as the direction of the fixed straight edge, the mica sheet can be turned over.

Figure 7 shows a might be formed in plugs. In this figure the manufacture of spark mica strip in this case is rotated about a cylinder the axis of which is perpendicular to the long axis of the mica strip. This is the usual situationas discussed above. It will be understood, however, that it is not necessary in order to practice this invention toout the mica into strips. This in vention applies generally to the bending of sheet mica about'a comparatively short radius.

Instead of relying upon the relationship between the pressure axes and the percussion axes to assist in determining the direct-ion of the major percussion axis, it may in some cases be found desirable to employ photoelectric means. I have shown means in Figure. 8. In this figure are shown the light polarizing substances I8 and 26, the light sources 20, a substantially transparent rotatable support 38 for the mica sheet which is to be oriented, and a photoelectric cell 38 which is connected in series relation with a galvanometer l0 and a variable resistance 42. An apparatus such 8 may be used todetermine exactly the position sion from which may be determined the direction mica cigarette such as,

is shown an elongated mica strip 32 which is rolled about a cylinder 34. The

a diagrammatic arrangement of such of minimum light transmispercussion lines therein,

parallel to the mica is to be bent or rolled may vary 15 to one side of the theoretically best rolling direction.

. Specifically, referring to Figure 1, it may be found that the direction of the cylinder may be 15 clockwise from axis H or 15 counterclockwise from axis y-y without substantially impairing the workability of-themica. It will generally be undesirable to go in the counterclockwise from axis ',a-e or clockwise from axis 11-11 because'moving in those directions will put the rolling axis too nearly parallel to the major percussion axis :r-zr.

It may also be found with some types of mica that, in the formation of several pressure figures, one pressure axis is always missing. This phenomenon might indicate for this grade of mica that the missing pressure axis is an axis of strength. Accordingly, in some cases it may be 'found satisfactory to bend or roll the mica about bending the mica sheets about the axis of least a weakness as indicated by the cracks.

2. The method of orienting a mica sheet to an optimum rolling axis comprising: subjecting the sheet to pressure to pressure weakness axes, and rolling the mica sheet about a cylinder whose axis is substantially perpendicular to one of said pressure axes.

3. The method of orienting mica sheets for bending comprising: subjecting one sheet of a book of sheets to pressure to formcracks, therein indicating pressure weakness axes, separating the several sheets of the book from one another and bending all the sheets of said book about a cylinder whose axis is substantially perpendicular to one of said pressure axes.

4. The method of orienting a mica sheet for bending comprising: stressing the sheet to form and bending the sheet about a cylinder whose axis makes an angle of from 60 to 75 with the strongest percussion line.

5. The method of claim 4, in which the strongest percussion line is determined optically by exposing the sheet to polarized light.

6. The method of claim 4, in which the sheet oriented is one of a book comprising a number of sheets all having the samecrystalline orientation and having substantially the same contour.-

7 A method of processing mica comprising the steps of determining the axes of weakness of one sheet of a book, separating the several sheets of the book from one another and bending all the sheets of the book about a cylinder whose axis is axis of least weakness.

8. The method of orienting mica comprising: forming pressure lines in a sheet of mica, exposing the sheet to polarized light to determine the approximate direction of the strongest percussion axis, and determining the direction of said axis precisely by aligning the pressure lines with the mica is who bent q other directions, 1. e.

form cracks therein indicating indicia. properly oriented on the source of the polarized light.

. 9. The invention of claim 8, and marking said sheet for cutting into elongated strips, the long axes of which make from 15 to 30 with said permission axis.

- 10. The invention of claim 8, and cutting said sheet into pieces for bending about a. cylinder 4- 2,344,670 whose axis makes from 60 to 75 with said percussion axis.

11. A cylinder comprising rolled sheet I'm. the axis of said cylinder making from 60 to 75 with 5 the direction of the principal percussion axis of the mica.

HELEN BLAIR BARLEIT. 

